Elevated levels of mechanical stress may induce tissue trauma by reducing perfusion or compromising structural integrity. When tissue is compressed such as when a growing tumor displaces the tissue, the capillary volume in the region is reduced. This, in general, causes a concomitant reduction in blood perfusion to the area. Additional compression may cause the development of nerve pinching, microtears and other structural defects. If the capillary pressure increases while the surrounding tissue is confined by rigid or indistensible walls trauma may also result. A good example of the latter is the case of cardiac or brain edemas where increased fluid volume occurs at the expense of decreased tissue volume. Either condition may cause damage to vital biological structures. This is also true of stresses applied to other parts of the human body. The stress developed within the tissue that causes this damage is called the mechanical contact stress. Contact stress is defined as the normal component of the interfacial stress between two bodies. It should be remarked that contact stress within the context of this invention is not the hydrostatic pressure.
In situations where the monitoring of pressure is necessary such as those encountered in the biomedical arts it is desirable to have a device which is entirely implantable within the body of the subject. Many prior art devices provided for this purpose are transcutaneous, that is, a portion of the device extends outside of the subject's body. These devices have the drawback that they have a high incidence of infection at the point where the device emerges from the subject's body. In addition, in many instances cables emerging from the subject's body limit the mobility of a patient (e.g., U.S. Pat. Nos. 4,660,568, 4,246,908, 4,393,878, 4,186,749 and 4,738,267, French Pat. Nos. 2,455,735, 2,384,482, 2,274,261 and 2,262,953 and German Pat. No. 1,965,231, among others).
Other prior art devices are known which are fully implantable and do not require any connectors to emerge through the skin of a subject's body. These devices are implantable electronic devices which are interrogated by induction or which transmit coded information to an appropriate monitor. In many instances they contain complex precision electronic equipment which must be implanted, e.g., inside the head of a subject, and require high expenditures, and sophisticated monitoring apparatuses (e.g., U.S. Pat. Nos. 3,977,391, 4,124,023, 4,006,734, 3,911,902 and 2,566,369, among others).
Other devices are also known which utilize radiopaque materials which are mechanically shifted in proportion to changes in pressure once the device is implanted. One such example is the X-ray opaque device described in U.S. Pat. No. 4,627,443. Another such device is described in U.S. Pat. No. 4,660,568. This patent provides an intracranial implantable sensor which undergoes a conformational change with pressure and is coupled through the skin by electromagnetic, acoustic or mechanical transmission to an external device which detects the change and interprets the pressure. This sensor has an insulating body with a moveable element which oscillates along an opening or channel in the body of a subject. The element communicates with the external atmospheric pressure by means of a membrane which is nearly coplanar with the intact skin covering it and on the other side with the internal pressure by another membrane. The degree of the moveable element's displacement relative to the body of the subject is correlated with the difference in the internal and atmospheric pressures.
A further type of implantable pressure sensor element is that provided by U.S. Pat. No. 3,958,562. This sensor element comprises a silicone rubber vessel, the walls of which have wax trapped therewithin, which is filled with a non-toxic hydraulic fluid compatible with body fluids. However, this pressure sensor has attached thereto a piece of transcutaneous tubing for conducting out displaced liquid which may operate as a bellows.
A further implantable device is that described in U.S. Pat. No. 4,340,038 which operates as a magnetic field concentrator and may function for instance as an intracranial pressure monitoring device. The implanted sensor has a magnetic field pick-up which when placed next to an external magnetic field generator is capable of converting magnetic energy to electrical energy for energizing the device. The device contains a ferrite material and is adapted to operate in response to energy provided by an externally located magnetic field generator.
Thus, there is still a need for further development of biocompatible implantable devices which are capable of fast measuring of contact pressure by non-invasive means.